The automotive industry, in order to reduce the weight of automobiles, has increasingly substituted aluminum alloy panels for steel panels. Lighter weight panels, of course, help to reduce automobile weight, which reduces fuel consumption, but the introduction of aluminum alloy panels creates its own set of needs. To be useful in automobile applications, an aluminum alloy sheet product must possess good forming characteristics in the as-received T4 temper condition, so that it may be bent or shaped as desired without cracking, tearing or wrinkling. At the same time, the alloy panel, after painting and baking, must have sufficient strength to resist dents and withstand other impacts.
Several aluminum alloys of the AA (Aluminum Association) 2000 and 6000 series are usually considered for automotive panel applications. The AA6000 series alloys contain magnesium and silicon, both with and without copper but, depending upon the Cu content, may be classified as AA2000 series alloys. These alloys are formable in the T4 temper condition and become stronger after painting and baking. Because thinner and therefore lighter panels are required, significant increases in strength after painting and baking will be needed to meet these requirements.
In addition, known processes for making sheet material suitable for automotive panels from the alloys has involved a rather complex and expensive procedure generally involving semi-continuous direct chill (DC) casting of the molten alloy to form an ingot, scalping of the ingot by about 1/4 inch per rolling face to improve the surface quality, homogenizing the alloy at a temperature between 500.degree. to 580.degree. C. for time periods between 1 to 48 hours and hot and cold rolling to the desired gauge. The rolled material may then be given a solution heat treatment at 500.degree. to 575.degree. C. for 5 minutes or less in a continuous heat treatment line, rapidly quenched and naturally aged for 48 hours or more. In this procedure, the scalping and homogenizing steps are particularly troublesome. Moreover, the homogenizing step prevents the sheet from being produced essentially continuously from the casting step to the re-roll step following hot rolling.
There is therefore a need for improved alloys and for improved processes for fabricating sheet material from such alloys.